The present invention relates to a technique for transmitting/receiving radio signals and particularly to a technique effectively applied to the improvement of receiving sensitivity in a cellular phone and the like which use a direct conversion system.
In recent years, a cellular phone has been in widespread use as a device for mobile communication, and there have been increasing demands for a wide variety of functions in the cellular phone. With an aim to reduce the number of electronic parts and the area of a printed wiring board, some cellular phones employ a direct conversion system in a receiving unit of a semiconductor integrated circuit device for a RF (radio frequency) processing.
In the direct conversion system, small signals with a band of 800 MHz to 2 GHz, received by an antenna, are directly converted into signals with a voice band (of about several tens KHz) by the use of one mixer circuit.